1- There have been many different Portastudios. If you think you can get by with only 4 tracks, some good machines to consider from the used market are the Tascam 424, 464, and 644. If you want something a little more portable, check out the Tascam 414.
Should you need 8 tracks, the best unit is the 688, although it's a little more complicated to use, so make sure you get a manual for it. The other option for 8 tracks on cassette in a Portastudio format is the Tascam 488.
2- You shouldn't need an external pre-amp, with a few exceptions:
a- You need phantom power for your mics, at which point it may be worth it to invest in a better pre-amp that provides phantom power as well.
b- You're using one of the Portastudios that only has 1/4" inputs, rather than XLR. However, with the correct adapter, an external pre-amp would not be needed.
The 424, 464, 644, and 688 all have XLR inputs.
-MD